This invention relates generally to methods and apparatus for monitoring optical emissions from a flame within the combustor stage of a gas turbine engine through a window and, more particularly, to a method and apparatus for compensating for degradation in the transmission of optical emissions due to coatings on the window.
Gas turbines are extensively used as power plants for a wide diversity of applications including, as examples, electric power generators in utility power plants, land based engines for gas fired electrical generator or pipeline compressors, and shipboard or airborne engines for, respectively, marine or aeronautical propulsion.
Gas turbines burn hydrocarbon fuel which may include natural gas or kerosene, which is used as an aviation (jet) fuel. As a result of the combustion process, such turbines emit an exhaust stream containing a number of combustion products, including various forms of nitrogen oxide, collectively referred to as "NO.sub.x ", which is considered a pollutant.
It is widely known that, for a gas turbine, NO.sub.x emissions increase significantly as the combustion temperature rises. It is also known that operating a turbine in a so-called "lean burn" condition, which involves use of a lean mixture of fuel and combustion air (i.e., a relatively low fuel-to-air ratio), reduces the combustion temperature to a level that significantly reduces NO.sub.x emissions.
Brown et al. U.S. Pat. No. 5,257,496, issued Nov. 2, 1993, entitled "Combustion Control for Producing Low NO.sub.x Emissions Through Use of Flame Spectroscopy" and related Brown patent application Ser. No. 08/226,528, filed Apr. 12, 1994, also entitled "Combustion Control for Producing Low NO.sub.x Emissions Through Use of Flame Spectroscopy", both of which are assigned to the instant assignee, disclose closed loop feedback control systems which achieve a lean burn by employing a silicon carbide (SiC) photodiode to sense combustion temperature through measurement of the intensity of ultraviolet radiation from a combustion flame and continuously adjusting the fuel/air ratio of the fuel mixture such that the ultraviolet radiation intensity remains below a predetermined level associated with a desired low level of NO.sub.x emissions. Photocurrent produced by the photodiode is proportional to the photon flux produced by the flame and impinging on the photodiode.
The SiC photodiode itself is not actually located within the gas turbine combustor. Rather, an optical window is provided in a wall of the combustor, thus separating a combustion side region from a non-combustion side region, and the SiC photodiode or other suitable detector "views" the combustion flame through this window.
The window surface on the combustion side has the potential for becoming coated, over time, with materials such as carbon as a result of the high temperature, high pressure combustion process, thereby effectively degrading the transmission of optical emission from the combustion flame through the window.
While such degradation is not of particular concern in a simple on/off flame detection application, a potential problem is presented when optical emissions from the combustion flame are sensed for more sophisticated control purposes, such as the sensing of flame intensity as disclosed in Brown et al U.S. Pat. No. 5,257,496 and Brown patent application Ser. No. 08/226,528, with control of the combustor fuel-to air ratio being based in part on the sensed emission intensity. As another example, Shu et al. application Ser. No. 08/339,843, filed concurrently herewith, entitled "Optical Sensing of Combustion Dynamics", discloses a system wherein AC components at acoustic frequencies in the photodiode detector output signal, which correspond to various combustion dynamic frequencies, are recognized for monitoring and controlling combustion dynamics.